Uncoupled O and Hf isotopic systems in zircon from the contrasting granite suites of the New England Orogen, eastern Australia: Implications for studies of Phanerozoic magma genesis

摘要

The Permo-Triassic granites of the New England Orogen, eastern Australia, were emplaced into a volcanic arc complex accreted to the eastern Gondwana margin in the Late Devonian or Early Carboniferous. Zircon U-Pb dating shows that the S-type Hillgrove (similar to 297 Ma) and Bundarra (similar to 287 Ma) Supersuites predated intrusion of the I-type Moonbi Supersuite (similar to 250 Ma) by up to 50 Ma. The high delta O-18(zrn) of the S-type granites (10.0-11.5 parts per thousand), and range of U-Pb ages (similar to 370-300 Ma) and delta 18O(zrn) (similar to 5-10 parts per thousand) of their inherited zircon cores, show that their source rocks were predominantly weathered Carboniferous volcaniclastics, the youngest deposited <25 Ma before the granites were emplaced. In contrast, the lower delta O-18(zrn) (6.9-7.8 parts per thousand) and lack of inheritance in the I-type granites is consistent with a zircon poor, more juvenile source, probably a mafic igneous underplate mixed with a small amount of volcanogenic and/or oceanic sediment. Despite the differences in source materials, the epsilon Hf(t) values of all granites, both S- and I-type, are similar (+5.0 +/- 0.5 cf. +5.9 +/- 0.5), consistent with both the mafic and sedimentary components in the granite sources being relatively young and similar in Hf isotopic composition at the time of granite genesis. In young, isotopically juvenile orogens, the O isotopic composition of well-dated igneous and inherited zircon can be a much more sensitive indicator of petrogenetic processes than the zircon Hf isotopic compositions alone. (C) 2014 Elsevier Ltd. All rights reserved.